1. Field of the Invention
The present invention relates to a method for holding a sheet material wound around a circumferential surface of a rotating drum. The invention further relates to an image recording apparatus disposed with a rotating drum around which a sheet-like recording material can be wound, with an image being recorded on the recording material wound around the rotating drum while the rotating drum and a recording section disposed in correspondence to the circumferential surface of the rotating drum are relatively moved.
2. Description of the Related Art
Technology has been developed in which, using a sheet-like recording material, and particularly a printing plate precursor including a photosensitive layer formed on a substrate, an image is recorded directly with a laser beam or the like on the photosensitive layer (an emulsion surface) of the printing plate precursor (printing plate exposure apparatuses). With such technology, images can be rapidly recorded on printing plate precursors.
In an automatic printing plate exposure apparatus using such technology for recording an image on a printing plate precursor, the image is recorded on the printing plate precursor wound around a circumferential surface of a rotating drum, while the drum is rotated at a high-speed (main scanning) and an exposure head is moved along an axial direction of the rotating drum (sub scanning).
Among this type of automatic printing plate exposure apparatus, an apparatus is known in which one end of the printing plate precursor is clamped onto the rotating drum by a clamp mechanism including an open-close chuck that is fixed or removably disposed at the rotating drum.
The clamp mechanism holds the printing plate precursor to the rotating drum by first opening the chuck (to create a space between the chuck and the rotating drum) to allow the printing plate precursor to be fed onto the rotating drum from a direction tangential to the rotating drum and then closing the chuck (to thereby nip the printing plate precursor between the chuck and the rotating drum) when the fed printing plate precursor has been registered at a predetermined position.
However, slack, resulting from waves or curls already present in the printing plate precursor prior to the printing plate precursor being nipped against the rotating drum, is sometimes generated when the printing plate precursor is nipped, due to the chuck simultaneously closing across the width direction of the printing plate precursor. This leads to degradation of image quality, because the accuracy with which the printing plate precursor is registered on the rotating drum drops and because the printing plate precursor is not in close contact with the circumferential surface of the drum.
In view of the aforementioned, the present invention has been designed to eliminate slackening of a sheet material (particularly a recording material such as a printing plate precursor) wound around a rotating drum, which slackening is caused when a leading edge and/or a trailing edge of the sheet material is held by a chuck when the sheet material is fed from a direction tangential to the rotating drum and wound around the drum. Thus, objects of the invention are to provide a method for holding a sheet material and to provide an image recording apparatus, wherein the accuracy with which a recording material is registered can be improved and degradation of image quality can be prevented.
A first aspect of the invention is a method for holding a sheet material wound around a circumferential surface of a rotating drum, comprising the steps of: (a) feeding the sheet material to the circumferential surface of the rotating drum from a direction tangential to the circumferential surface of the rotating drum; and (b) nipping at least one of a leading edge and a trailing edge of the fed sheet material between a chuck and the circumferential surface of the rotating drum, wherein, when the sheet material is nipped between the circumferential surface of the rotating drum and the chuck, the sheet material is sequentially nipped from one axial-direction end of the rotating drum to the other axial-direction end of the rotating drum.
Even when the sheet material is wavy, the wavy sheet material can be sequentially flattened out from one axial-direction end of the rotating drum to the other axial-direction end of the rotating drum, due to the chuck sequentially nipping the sheet material from one axial-direction end of the rotating drum to the other axial-direction end of the rotating drum. Therefore, slackness in the sheet material can be eliminated when the nipping is completed.
A second aspect of the invention is an image recording apparatus for recording an image on a sheet-like recording material by relative movement between a rotating drum which includes a circumferential surface, on which the recording material is wound, and a recording section disposed in correspondence to the circumferential surface of the rotating drum, comprising: clamp mechanisms disposed at an area on the circumferential surface of the rotating drum along an axial direction of the rotating drum, each clamp mechanism comprising a chuck that nips a leading edge of the recording material, which is fed from a direction tangential to the circumferential surface of the rotating drum, to the circumferential surface of the rotating drum; a moving mechanism, disposed so as to correspond to the chucks, for selectively moving the chucks into a nipping position, at which the recording material is nipped, or into a nip-release position, at which the recording material is released from being nipped; and a time-difference mechanism for moving the chucks into the nipping position in sequential order from the chuck disposed nearest one axial-direction end of the rotating drum to the chuck disposed nearest the other axial-direction end of the rotating drum.
In the image recording apparatus having the above-described structure, the recording material is fed in the direction tangential to the rotating drum. At this time, the chucks of the clamp mechanisms are set in the nip-release position by the moving mechanism, and the recording material is interposed between the chucks and the circumferential surface of the rotating drum.
When the interposed recording material is registered in a predetermined position, the moving mechanism moves the chucks into the nipping position.
At this time, the moving mechanism does not move the chucks arranged along the axial direction of the rotating drum into the nipping position all at once, but moves the chucks in sequential order into the nipping position from one axial-direction end of the rotating drum to the other axial-direction end of the rotation drum using the time-difference mechanism. In this manner, even when the recording material is wavy, the waviness of the recording material can be sequentially flattened out toward the other end side, whereby slackness in the recording material can be eliminated when the nipping by all of the chucks is completed.
In the second aspect, each clamp mechanism may comprise an urging member for urging the chucks toward the nipping position and a supporting post for supporting the chucks so that the chucks are pivoted by an urging force of the urging member.
The chucks can be rotated by the urging mechanism, with the supporting posts being the fulcrums, to maintain nipping of the recording material. Therefore, the recording material can be reliably held at the time of image recording.
Further, in the second aspect, the image recording apparatus may further comprise trailing edge clamp mechanisms disposed at an area on the circumferential surface of the rotating drum along the axial direction of the rotating drum, each trailing edge clamp mechanism comprising a trailing edge chuck that nips a trailing edge of the recording material, which is fed from a direction tangential to the circumferential surface of the rotating drum, to the circumferential surface of the rotating drum.
After the leading edge of the recording material is nipped by the chucks, the recording material is gradually wound around the circumferential surface of the rotating drum as the rotating drum rotates. Thereafter, the trailing edge of the recording material is also nipped by the trailing edge chucks of the trailing edge clamp mechanisms, whose structure is similar to that of the clamp mechanisms.
Yet in the second aspect, the image recording apparatus may further comprise a trailing edge chuck moving mechanism, disposed so as to correspond to the trailing edge chucks, for selectively moving the trailing edge chucks into the nipping position or into the nip-release position.
Still in the second aspect, the image recording apparatus may further comprise a time-difference mechanism for moving the trailing edge chucks into the nipping position in sequential order from the trailing edge chuck disposed nearest one axial-direction end of the rotating drum to the trailing edge chuck disposed nearest the other axial-direction end of the rotating drum.
Because the trailing edge chucks also sequentially nip the trailing edge of the recording material from one axial-direction end of the rotating drum to the other axial-direction end of the rotating drum, slackness in the recording material can be further eliminated.